The present invention relates to a method for the making of solder paste and to a solder joint obtained by this method.
The applications and performance characteristics of electronics are calling for increasingly efficient, compact and reliable electronic systems. These large-scale integration requirements are based on the use of miniaturized components and appropriate joining methods. The most widespread joining method entails the use of soldering to form joints between the component and the interconnection substrate. The soldered joint has three main functions: electrical interconnection, mechanical assembly and the transfer of heat from the component to the interconnection substrate which, most commonly, is a printed circuit. Now, there is a great difference between the coefficients of thermal expansion (CTE) of the electronic component and of the printed circuit (see FIG. 1). When there are temperature variations due to internal or external causes, the differences in expansion between the two elements induce strains that give rise to cracks. These cracks spread and cause the joint to break and the equipment is thus put out of operation.
An object of the invention is a solder paste that can be used to absorb the strains due especially to differences in CTE between the elements that it serves to join by soldering, and that does so throughout the lifetime of these elements, these strains possibly being high amplitude strains that can vary in substantial proportions, the joint that is made with this solder paste entailing no increase in cost, mass or volume, necessitating no modification of the architecture of the printed circuit, placing no limits on the performance characteristics of the circuits thus made, and permitting repairs
An object of the invention also is a method for the manufacture of a solder paste of this kind that is simple to implement and can be used to obtain a paste of homogeneous and constant quality.
The solder paste according to the invention comprises a metal matrix in whose volume particles of superelastic alloy are distributed substantially homogeneously, these particles having a substantially spherical shape with a diameter advantageously ranging from 1 to 50 xcexcm approximately and representing 5 to 30% of the total volume of the paste. These particles are advantageously made of SMA (shape memory alloy).
The method of the invention consists in incorporating substantially spherical particles made of superelastic material into a matrix of solder after having coated these particles with a material that is wettable by the material of the solder, these particles being advantageously coated with a flux.